1. Field of the Invention
This invention relates to quantum well structures, and in particular but not exclusively to such structures configured for use as optical modulators.
2. Related Art
In the broad field of optical signal processing there are many applications for high performance optical signal encoding and processing elements. For example, in high speed optical fiber communications systems, direct modulation of laser sources leads to undesirable wavelength shifts, "chirp", in the optical output of the laser. One way in which chirp may be avoided is to cease modulating the laser directly, optical modulation being achieved through use of a modulator in the optical path of the laser's output. In the generally less well developed area of optical signal processing, components such as logic gates, latches, and signal encoders are required. The bandwidth which optical signal processing and optical communications potentially offer means that there is a desire for components which operate at high speed, typically switchable at GHz rates, and preferably switchable at tens of GHz.
The present invention is concerned with such signal processing components, and in particular with modulators, which comprise quantum well structures. A quantum well is, in its simplest form, a double heterostructure, with a layer of low band-gap material sandwiched between two layers of higher band-gap material. Typically the layers all comprise semiconductors, for example the double heterostructure may consist of GaAs sandwiched between identical layers of AlGaAs. If the layer of low band-gap material is sufficiently thin, of the order of 100.ANG. or less, the energy levels in the valence and conduction bands becomes quantised, and the structure is referred to as a "quantum well".
While single quantum wells do exhibit measurable quantum effects, the intensity or strength of the effects can be increased by increasing the number of quantum wells. Several, typically tens or many tens or hundreds of quantum wells are formed in a multilayer structure, which structures are referred to as "multiple quantum wells", or "multiple quantum well" ("mqw") structures.
The basis behind the use of quantum well structures as modulators is that they can exhibit large changes in their optical absorption coefficient on the application of an electric field.
In devices such as QW modulators, utilising excitonic effects, the exciton of most significance is that involving the n=1 heavy hole. In this specification, unless the context clearly requires otherwise, we refer to the n=1 heavy hole exciton.
Our own interpretation of the accepted explanation of this phenomenon will now be given with reference to FIG. 19 which shows, schematically, the behaviour of a conventional quantum well structure. The structure will be assumed to consist of a pair of GaAlAs layers 2, 2' with a GaAs layer 1 therebetween. Thin solid lines 3 and 4 indicate respectively the valence band maximum in bulk GaAlAs and GaAs. Thin solid lines 5 and 6 similarly indicate the conduction band minimum in bulk GaAlAs and GaAs respectively. However because the GaAs layer is thin enough to provide quantum confinement of the electrons and holes there is an increase in minimum energy for them both. The new minima, the quantum well minima, for the electrons and holes are shown as broken lines 7 and 8 respectively. Note that in FIG. 19 electron energy increases towards the top of the Figure and hence hole energy increases as one moves down the Figure. With no applied field the resultant energy gap 9 is greater than that of the equivalent bulk GaAs. Typical probability density distributions of electrons and holes in the well are indicated by 10 and 11. The probability density distributions are pseudo-Gaussian and centered on the mid-point of the well.
FIG. 1b shows, schematically, the effect of applying an electric field across the layers of the well of FIG. 1a. With the field applied, the shape of the potential energy well seen by the electrons and holes changes dramatically. As a result of the changed band edge variation the probability density distribution of electrons follows the drop in minimum conduction band energy and hence moves to the right (the positive potential side) in FIG. 1b. Similarly, the hole distribution follows the fall in valence band minimum (hole) energy and hence moves to the left (the negative potential side) in FIG. 1b. The result is that the band gap shrinks. This is the so-called quantum-confined Stark effect. The change in band gap of course causes a shift in the absorption band edge, increasing absorption at lower photon energies (a red shift). Thus a quantum well device can be used as a modulator for wavelengths in the region of the band edge.
Unfortunately, in addition to the desired band edge shift there is a significant reduction in the absorption coefficient when a field is applied. In U.S. Pat. No. 4,826,295 the absorption coefficient for photon energies greater than the no-field band gap is estimated in one example to fall from about 2000 cm.sup.-1 in the no-field case to about 300 cm.sup.-1 in the with-field case. For photon energies just below the no-field band gap, but above the relevant with-field band gap, the absorption coefficient is estimated in U.S. Pat. No. 4,826,295 to rise from less than 10 cm.sup.-1 with no field to about 300 cm.sup.-1 with an applied field. While not preventing the use of quantum well structures in practical modulators, the fact that an absorption coefficient drop is associated with the desired field-induced band edge shift is nevertheless a disadvantage of known quantum well optical devices.
In European patent application 0324505 there is described a second-harmonic generator or frequency doubler which comprises a quantum well structure. According to 0324505, the conversion of radiation of angular frequency .omega. to radiation having an angular frequency 2.omega., in a non-linear material, has an efficiency proportional to the square of that material's non-linear receptivity .psi..sup.(2). In EP 0324505, a dipole moment is induced in a quantum well structure, .psi..sup.(2) of that structure being proportional to the size of the dipole moment. In the first embodiment in '505, the well comprises 120.ANG. of GaAs sandwiched between AlAs barrier layers. Several such wells, together with thicker, charge,,separating layers of AlAs, form the intrinsic region of a p-i-n structure. By applying an electric field across the layers, the centers of gravity of the wave functions of the electrons and holes in the well shift, creating a dipole moment. With an applied field of unspecified strength, .psi..sup.(2) of the aforementioned structure is said to be 400 times as large as that of LiNbO.sub.3, and 5000 times as large as that of KDP.
The angular frequency, .omega., of the radiation to be up-converted is chosen to satisfy the relation 2h.omega..congruent.Eg, where Eg is the bandgap of the well material (here GaAs, whose bandgap is 1.42 Ev at 300K).
In place of the AlAs in the barrier layers, Al.sub.x Ga.sub.1-x As can be used, and this permits the construction of a waveguiding quantum well structure, further increasing the efficiency of conversion.
As an alternative to the formation of a dipole as the result of an applied field, an embodiment is proposed in which a dipole is formed by varying the composition of the well from (InAs).sub.1-x (GaAs).sub.x to (GaSb).sub.1-y (GaAs)y across the well width, AlAs barrier layers being used. This structure is suggested to give second harmonic generation with an efficiency as high as that in the first embodiment described above. Also it is stated that this graded structure allows one to dispense with the electrodes and power source used in the first embodiment, since the application of an electric field is no longer necessary.
Further embodiments in '505 include a modulator for modulating the bias electric field, thereby modulating .psi..sup.(2) and hence modulating the harmonic wave.
In a further embodiment a filter is provided at the optical output of the qw structure, the filter passing the second harmonic wave ant blocking the low frequency input signal. This embodiment is also proposed for use in combination with the modulator or the graded-composition well.
Nowhere in '505 is there any suggestion that any advantage is to be obtained for applications other than second-harmonic generation by having a quantum well structure having a compositionally induced dipole.
Nowhere in '505 is there any suggestion that there is any merit in providing a quantum well structure having a compositionally induced dipole with electrodes. In this connection it should be noted that in '505 it is suggested that with a graded structure the dipole is fixed and hence .psi..sup.(2) is fixed, while modulating the bias voltage on a non-graded well structure--where .psi..sup.(2) is dependent on the size of the applied field, results in a desired modulation of the second harmonic signal through modulation of .psi..sup.(2).
In their paper in journal of Applied Physics, Vol. 62, No. 8, pp 3360-3365, Hiroshima and Nishi describe a graded-gap quantum well (GGQW) structure in which there is an effective `internal` electric field which concentrates the carriers on the same side of the well in both the conduction and valence bands. The authors note that such a structure can be realized by varying the alloy composition in the well layer so that the band gap varies linearly within the well. The paper deals with a theoretical analysis of the so-called quantum-confined Stark effect with particular attention being paid to excitonic effects. The structure on which their theoretical analysis is built is a GGQW comprising an Al.sub.x Ga.sub.1-x As well layer, 100.ANG. thick, in which the aluminum content x varies from 0 to 0.15 along the growth direction, and Al.sub.0.6 Ga.sub.0.4 As barrier layers. The authors note that the electron and light-hole envelope functions for various applied field conditions are nearly symmetric and are less effected than the heavy-hole envelope function by an external applied field.
It is interesting to note that Hiroshima and Nishi are concerned only with their linearly-graded gap quantum well structure, which in their words "has an effective "internal" electric field which concentrates the carrier on the same side of the heterointerface in both the conduction and valence bands". Nowhere do they suggest that pushing the electrons and holes to the same side of the well with a built-in field is a bad idea or that there is anything to be gained by building a structure in which the electrons and holes are pushed to opposite sides of the well by a built-in field. The authors are not concerned with establishing a structure which inherently produces electron-hole pairs as dipoles and do not teach towards such a concept. From our own study of the paper, it appears that the structure which they describe does produce a small inherent dipole, probably with a dipole moment of no more than 5.ANG..